Microwave circuit boards and method of manufacture thereof

ABSTRACT

Circuit boards, useful in microwave frequency applications, are formed from a laminate consisting initially of an insulator sandwiched between a copper foil and an aluminum plate. The boards are provided with through-hole connections by means of a procedure which includes drilling through the board to form through-holes, anodizing all exposed surfaces of the aluminum, electroplating copper over all anodized aluminum surfaces, sodium or plasma etching to alter the surface of the walls of the holes in the insulator to allow them to be &#34;wet&#34;, and electroless copper plating the entire board to form a continuous upper coating over the copper foil, the copper plate and the walls of the through-holes.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

The present invention relates to circuit boards which are suitable foruse at microwave frequencies. More specifically, this invention isdirected to circuit boards and a method of forming circuit boards havingthrough-hole connections in a laminate which includes a plastic filmsandwiched between copper and aluminum layers.

(2) Description of the Prior Art

The present invention has particular utility in the formation of circuitboards for transmission of electrical signals at microwave frequencies.Circuit boards for use at microwave frequencies often have acomparatively thick aluminum plate or layer which, among other uses,functions as a heat sink. A layer of plastic material, selected for itsdielectric and mechanical properties, will be adhesively bonded to onesurface of the aluminum layer and the circuit board laminate will alsohave a layer of copper foil bonded to the exposed surface of thedielectric film, i.e., the plastic layer. The dielectric film may, forexample, be comprised of polytetrafluoroethylene (PTFE) or fiberreinforced PTFE. This laminate of dielectric material sandwiched betweena relatively thick lower layer of aluminum and a relatively thin upperlayer of copper foil may be viewed as a preliminary, starter, orintermediate circuit board structure.

In such circuit boards it is necessary to establish electricalconnections between portions of the circuit, which will be formed on orin the copper foil layer, and the aluminum plate which forms part of theground plane. These connections are desirably implemented viathrough-holes in the laminate. The establishment of such through-holeconnections in a reliable and cost-effective manner is a problem of longstanding in the art. Most desirably, the through-hole connections willbe effected by electroless plating of copper. However, it is notpossible to produce, simply by electroless plating, a conductive pathbetween spaced copper and aluminum elements. It is, in fact, well knownin the art that plating upon aluminum requires extraordinary treatmentsto obtain the requisite adhesion.

There are presently two principal techniques for plating upon aluminum.These two techniques are the zincating process, wherein a thin layer ofzinc is deposited upon the aluminum base and other metals aresubsequently deposited over the zinc, and the anodizing process where aporous anodic coating is produced and a metal subsequently plated overthe coating. Because of its comparative ease of implementation andrelatively low cost, the zincating process is by far the more widely usetechnique. However, this technique has the disadvantages that it is verysensitive to the condition of the surface of the aluminum thus requiringtime consuming pre-zincating cleaning and etching treatments. Further,it is generally considered inadvisable to use zinc coated aluminummembers in electrical circuit applications since the zinc will meltduring soldering, mix with the solder and produce high resistance orotherwise defective connections. The foregoing is also true of cadmiumwhich is sometimes substituted for zinc.

Anodizing processes have not been employed in the production ofmicrowave circuit boards since it has been universally believed that thecopper would prevent an anodic coating from forming on the aluminumsurface or the copper would be chemically attacked and thereby renderedunfit for use.

Accordingly, when faced in the prior art with the necessity ofestablishing a through-hole connection between the aluminum and copperlayers of a microwave circuit board laminate, and being unable to employa zincating process, resort has been had to the unreliable technique oftrying to deposit copper or nickel either directly on the aluminum or onaluminum which has previously been given a chromate conversion coating.As is well known, such methods produce poor quality plated metalcoatings that exhibit low bond strength to the aluminum base metal.Thus, exposure of these poor quality coatings to elevated temperatures,for example soaking for one hour at 250° F. or immersion in molten 60/40tin/lead solder, will cause the coating to blister.

SUMMARY OF THE INVENTION

The present invention overcomes the above-discussed and otherdeficiences and disadvantages of the prior art by providing a novelmicrowave circuit board laminate with plated through-holes and byproviding a novel method of manufacturing circuit boards from suchlaminates.

The article and process of the present invention involves microwavecircuit boards with conductive through-holes. The invention starts witha laminate structure of a sheet of dielectric material with a thin layerof copper on one surface and a thicker layer of aluminum on the othersurface. Through-holes are drilled through the laminate in apredetermined pattern. All exposed aluminum surfaces (the layer ofaluminum and the walls of the through-holes passing through the aluminumlayer) are anodized by a carefully controlled process to define a porousaluminum oxide coating. During the anodizing process the thin copperlayer on the dielectric is electrically isolated from the aluminum.Next, a layer of conductive material is electroplated over all of thealuminum oxide. Next, the structure is subjected to a sodium or plasmaetch to treat the walls of the holes in the dielectric to allow them tobe "wet". Next, the entire structure is electroless plated to form acomplete coating of metal on all exposed surfaces of the structure,including all exposed electroplated metal areas, the original thincopper layer and the entire through-hole surfaces. Thereafter amicrowave circuit pattern will be defined on the surface whichoriginally had the thin layer of copper, with the plated through-holesthen forming conductive contacts between the circuit pattern and theother copper surface (over the aluminum) which is the ground plane.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a flow diagram of the principal steps of the process of thepresent invention.

FIGS. 2A-2E show the circuit board at various process steps.

It is to be understood in the drawings of FIGS. 2A-2D that the showingsof various metal coatings are shown for illustration purposes only andare not to scale or proportion.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the preferred embodiment, microwave circuit boardsare fabricated from a preliminary or intermediate laminate consistingoriginally of a copper foil 10 and an aluminum plate 12 adhesivelybonded to opposite surfaces of a sheet of fiber reinforcedpolytetrafluoroethylene 14 (see FIG. 2A). The laminate may, for example,comprise "RT/duroid" 5870 or 5880 available from Rogers Corporation,Rogers, Connecticut. In one reduction to practice the copper foil was0.0014 inches thick, the sheet of insulating material was 0.060 inchesthick and the aluminum layer was 0.065 inches thick and was comprised of6061-T6 type aluminum.

After formation of the preliminary laminate (step A), the next step(step B) in the fabrication of a microwave circuit board comprises thedrilling of through-holes 16 of the required dimensions and at thedesired points through the laminate (see FIG. 2B). The drilled holes arethen deburred if necessary. The laminate is then cleaned. Cleaning mayexpeditiously be accomplished employing a cleaning solution which isneutral with respect to both the aluminum and copper. The cleaning stepis for the purpose of degreasing the laminate and removing ordinary shopsoil and fingerprints.

The cleaned laminate is then immersed in an aqueous anodizing solutionwhich, in one reduction to practice of the invention, consisted ofthirty (30%) percent by volume of eighty-five (85%) percent H₃ PO₄. Allexposed aluminum surfaces (i.e., all originally exposed surfaces ofaluminum and the interior walls of the portions of the through-holespassing through the aluminum layer) are anodized (Step C) by means of acarefully controlled process wherein electrical contact is made to thealuminum surface through the use of a reusable titanium clip. Care istaken to insure that the clip is electrically insulated from the copperside of the laminate. Failure to observe this precaution will result inthe copper being etched from the laminate and no anodic coating formedon the aluminum. During the anodizing process the temperature of thesolution is maintained within the range of 105° to 118° F. andpreferrably at 110° F. In one reduction to practice, where thetemperature of the solution was 110° F., the anode current density was25 amp/ft² and the anodizing was allowed to continue for five minutes.During anodization the solution was moderately agitated so as to insurethat oxygen bubbles which form on the surface of the aluminum duringanodization do not prevent complete anodization, especially on the wallsof the drilled holes. The oxide coating formed in the practice of thisinvention will have a coating weight in the range of 0.1 to 2.0milligrams/in².

Subsequent to the anodizing step the substrate is rinsed, typically bywashing in clean water.

The next step in the process (Step D) comprises the electroplating of acopper coating 18, or some other electrically conductive metal such asnickel, over the aluminum oxide coating formed during the anodizing step(see FIG. 2C). In the case of copper electroplating, the substrate wasimmersed in an acid CuSO₄ solution until the aluminum oxide was coveredwith an adherent copper film. Due to the porosity of the aluminum oxidecoating produced through careful control of the anodizing step, thecopper electroplated over the aluminum oxide will penetrate the poresand, it is believed, will also plate on the aluminum base metal. In anyevent, the plated copper will strongly adhere to the oxide coating andwill be in electrical contact with the base aluminum whereby anelectrical connection to the base metal may be established by makingelectrical contact with the copper plate. In one reduction to practice,the copper was placed from a CuSO₄ --H₂ SO₄ solution obtained fromLea-Ronal Inc., Freeport, New York and sold commercially under the nameCopper Gleam PCM. In another reduction to practice, a nickel plate wassubstituted for the copper and a (NH₂ SO₃ H)₂ Ni solution was utilized.The plated metal serves as the ground plane in the resulting microwavecircuit board.

It has been found particularly important, in order to obtain a coatingwhich will reliably establish electrical contact with the aluminum basemetal, that the DC power should be on when the laminate is inserted inthe plating tank. Typical parameters of a copper plating step are asfollows:

solution temperature=70° to 90° F.

cathode current=20 to 40 amps/ft²

anode-cathode area ratio=1:1 to 2:1

time=10 to 20 min.

Solution agitated with clean air (bubbling) The thickness of the platingwill be in the range of 0.0002 to 0.0004 inches for both copper andnickel.

After the electroplating step, the structure is subjected to a sodium orplasma etching (Step E) to alter all exposed surfaces of the dielectric,especially the walls of the through-holes. This treatment allows thewalls of the holes to be wetted, i.e., accept or receive a water basedmaterial.

Subsequent to the etch treatment, the laminate is washed with acatalyzing (paladium salt) bath. Then, the structure is electrolesscopper (or nickel) plated to form a continuous copper (or nickel)coating 20 over the original copper foil, the copper plate (on theoriginal aluminum) and the walls of the through-holes. (See FIG. 2D andFIG. 2E, which is a segmented section taken along line E-E of FIG. 2D)The copper (or nickel) plating on the walls of the through-holesestablish conductive paths through the drilled holes between the thincopper foil and the copper plating and, via the plated copper to thealuminum base metal. The microwave circuit board structure is thencompleted and ready for formation or definition of a circuit patternthereon.

After this circuit board structure has been completed, one or moremicrowave circuit patterns may be formed on the surface carrying theoriginal thin copper foil. Portions of the circuit pattern will, asdesired, be connected to the ground plane by the now conductively coatedthrough-holes.

As will be obvious to those skilled in the art, it is within thecontemplation of the present invention to apply additional metaldeposits such as, for example, copper-solder or copper-nickel-gold forthe purpose of enhancing electrical properties and/or solderability ofthe copper layer or to obtain metal layers which are resistant to theaction of chemical etchants used to define circuit elements.

While a preferred embodiment has been described, various modificationsand substitutions may be made thereto without departing from the spiritand scope of the invention. Accordingly, it will be understood that thepresent invention has been described by way of illustration and notlimitation.

What is claimed is:
 1. A method of preparing a circuit board from alaminate of dielectric material sandwiched between layers of copper andaluminum comprising the steps of:forming at least one through-hole inthe laminate; anodizing the exposed areas of the aluminum layer in H₃PO₄ while electrically isolating the aluminum from the copper;electroplating a conductive metal over the anodic coating on thealuminum; etching exposed areas of the dielectric material; andelectroless plating a conductive metal coating over all exposed surfacesof the laminate.
 2. The method of claim 1 wherein the step of formingthrough-holes further comprise:forming a pattern of holes through thealuminum-dielectric-copper laminate whereby the subsequent steps ofanodizing, electroplating, etching and electroless plating willestablish conductive paths through the board via plated through-holes.3. The method of claim 1 wherein the step of anodizingcomprises:establishing electrical contact to the aluminum layer by meansof a titanium contact electrically isolated from the copper layer. 4.The method of claim 3 wherein the step of electroplatingcomprises:immersing the anodized laminate in a plating tank containingCuSO₄ subsequent to establishing electrical contact between a currentsource and the anode and cathode of the plating apparatus.
 5. The methodof claim 1 wherein the step of anodizing includes:maintaining thetemperature of the H₃ PO₄ in the range of 105° to 118° F.
 6. The methodof claim 3 wherein the step of anodizing includes:maintaining thetemperature of the H₃ PO₄ in the range of 105° to 118° F.
 7. The methodof claim 4 wherein the step of anodizing includes:maintaining thetemperature of the H₃ PO₄ in the range of 105° to 118° F.
 8. The methodof claim 1 wherein the step of anodizing is continued until an oxidecoating having a weight in the range of 0.1 to 2.0 milligrams per squareinch is formed.
 9. The method of claim 7 wherein the step of anodizingis continued until an oxide coating having a weight in the range of 0.1to 2.0 milligrams per square inch is formed.
 10. The method of claim 1wherein the step of electroplating includes electroplating copper ornickel under the following conditions:solution temperature=70° to 90° F.cathode current=20 to 40 amps/ft² anode-cathode area ratio=1:1 to 2:1time=10 to 20 min. solution agitated with clean air.
 11. The method ofclaim 1 wherein the step of electroplating includes:immersing thelaminate in an electroplating tank for which the power is on prior toimmersion.
 12. The method of claim 11 wherein the step of electroplatingincludes electroplating copper or nickel under the followingconditions:solution temperature=70° to 90° F. cathode current=20 to 40amps/ft² anode-cathode area ratio=1:1 to 2:1 time=10 to 20 min. solutionagitated with clean air.
 13. The method of claim 1 including the stepsof:cleaning the laminate after formation of said through-hole.
 14. Themethod of claim 1 including the steps of:cleaning the laminate with asolution neutral to both aluminum and copper after formation of saidthrough-hole.
 15. The method of claim 1 including the steps of:rinsingthe substrate after the step of anodizing.
 16. A method of preparing acircuit board from a laminate of dielectric material sandwiched betweenlayers of copper and aluminum comprising the steps of:forming at leastone through-hole in the laminate; anodizing the exposed areas of thealuminum layer in an anodizing solution while electrically isolating thealuminum from the copper; electroplating a conductive metal over theanodic coating on the aluminum; etching exposed areas of the dielectricmaterial; and electroless plating a conductive metal coating over allexposed surfaces of the laminate.
 17. The method of claim 16 wherein thestep of forming through-holes further comprise:forming a pattern ofholes through the aluminum-dielectric-copper laminate whereby thesubsequent steps of anodizing, electroplating, etching and electrolessplating will establish conductive paths through the board via platedthrough-holes.
 18. The method of claim 16 wherein the step of anodizingcomprises:establishing electrical contact to the aluminum layer by meansof a titanium contact electrically isolated from the copper layer. 19.The method of claim 18 wherein the step of electroplatingcomprises:immersing the anodized laminate in a plating tank containingCuSO₄ subsequent to establishing electrical contact between a currentsource and the anode and cathode of the plating apparatus.
 20. Themethod of claim 16 wherein the step of anodizing includes:maintainingthe temperature of the H₃ PO₄ in the range of 105° to 118° F.
 21. Themethod of claim 18 wherein the step of anodizing includes:maintainingthe temperature of the H₃ PO₄ in the range of 105° to 118° F.
 22. Themethod of claim 19 wherein the step of anodizing includes:maintainingthe temperature of the H₃ PO₄ in the range of 105° to 118° F.
 23. Themethod of claim 16 wherein the step of anodizing is continued until anoxide coating having a weight in the range of 0.1 to 2.0 milligrams persquare inch is formed.
 24. The method of claim 22 wherein the step ofanodizing is continued until an oxide coating having a weight in therange of 0.1 to 2.0 milligrams per square inch is formed.
 25. The methodof claim 16 wherein the step of electroplating includes electroplatingcopper or nickel under the following conditions:solution temperature=70°to 90° F. cathode current=20 to 40 amps/ft² anode-cathode area ratio=1:1to 2:1 time=10 to 20 min. solution agitated with clean air.
 26. Themethod of claim 16 wherein the step of electroplating includes:immersingthe laminate in an electroplating tank for which the power is on priorto immersion.
 27. The method of claim 26 wherein the step ofelectroplating includes electroplating copper or nickel under thefollowing conditions:solution temperature=70° to 90° F. cathodecurrent=20 to 40 amps/ft² anode-cathode area ratio=1:1 to 2:1 time=10 to20 min. solution agitated with clean air.
 28. The method of claim 16including the steps of:cleaning the laminate after formation of saidthrough-hole.
 29. The method of claim 16 including the step of:cleaningthe laminate with a solution neutral to both aluminum and copper afterformation of said through-hole.
 30. The method of claim 16 including thestep of:rinsing the substrate after the step of anodizing.